This invention relates to a refrigeration system for vehicles, and more particularly to the construction of storage compartments for utilizing carbon dioxide as a refrigerant in transporting products by vehicles such as trucks, trailers, rail cars and the like.
Some mechanical systems utilizing expanded carbon dioxide gas as the refrigerant have been proposed heretofore. One such system is disclosed in Rubin U.S. Pat. No. 3,561,266 issued Feb. 9, 1971. In the Rubin patent, carbon dioxide gas was expanded within containers at the top of the vehicle storage area so as to form a deposit of snow within the containers. The snow provided a cold surface on the bottom of the containers to cool the circulating air within the vehicle. In addition, in one embodiment a single vent was provided in each container to permit carbon dioxide gas to flow into the storage area. In another embodiment, the carbon dioxide gas was vented to the atmosphere outside of the container.
In U.S. Pat. No. 4,502,293, issued Mar. 5, 1985, to Paul R. Franklin Jr., a carbon dioxide cooling system is shown wherein carbon dioxide snow is deposited within a compartment positioned at one end of the vehicle and within a passageway positioned just beneath the top wall of the vehicle. Fans are provided to draw air around the containers in the end compartment and through the passageway over the carbon dioxide snow so that the cooled air would circulate through the vehicle and maintain the contents at the desired cold temperature.
Other patents disclosing the use of carbon dioxide either in the form of blocks or snow within containers which are not open to the vehicle include: U.S. Pat. No. 1,975,177, the Sherrick U.S. Pat. No.; 2,325,371, Clerc U.S. Pat. No.; 2,731,807, Allyne U.S. Pat. No.; 3,783,633, Glynn et.al U.S. Pat. No.; 4,248,060, Franklin, Jr. U.S. Pat. Nos.; 4,299,429, 4,376,511, 4,381,649, and 4,404,818.
The American Frozen Food Institute conducted a study program trying to develop a suitable cyrogenic system for shipping frozen foods in rail cars. In a Executive Summary Report dated March 1985, one prototype railcar is described wherein liquid carbon dioxide is stored in a series of elongated tanks spaced lengthwise to the car beneath an aluminum floor. The liquid carbon dioxide was vented onto the top of the load to form a blanket of carbon dioxide snow on the top and around the load. The blanket was rebuilt as necessary during shipment.
Because of the direct contact of the snow with the load, the products in some parts of the load were reduced to below minus ninety degrees F. At such a low temperature, some products became very brittle and broke. Also, because the loads are giving off moisture the direct exposure of the snow making nozzles to the moisture caused them frequently to freeze closed. And the weight of the carbon dioxide tanks took away usable payload.
Fink et al, U.S. Pat. No. 4,593,536, represents a further development of the American Frozen Food Institute program. Therein a rail car is illustrated wherein carbon dioxide snow is deposited in a bunker at the roof of the car. Vents are provided along one sidewall for the escape of carbon dioxide so that it can flow downwardly along such side wall, through lateral passageways beneath the floor, and thence upwardly along the opposite sidewall and back across the load, all by convection. However, in practice the cold carbon dioxide gas did not effectively flow upwardly along the opposite side wall and the product adjacent such wall in a car so designed would get undesireably warm, particularly in warm weather.